GitHub Actions: return support for 32-bit builds

It was removed in 20947ed just due to GLEW package is missing.
Now, thanks to GLAD (#1684), we can return it.

.github/workflows/cibuild.yml

@@ -90,6 +90,7 @@ jobs:
  # Both cc and cxx should be set if we want to change the compiler.
  # You may also want to set XRAY_LINKER when changing the compiler.
  - { name: Ubuntu, os: ubuntu-latest, arch: amd64, cc: gcc, }
+ - { name: Ubuntu, os: ubuntu-latest, arch: i386, cc: gcc, }
  - { name: Ubuntu, os: ubuntu-latest, arch: amd64, cc: clang, cxx: clang++, flags: "-DXRAY_LINKER=lld", }
  #- { name: Ubuntu, os: ubuntu-latest, arch: arm64, cc: gcc, container: 'dockcross/linux-arm64', }
  #- { name: Ubuntu, os: ubuntu-latest, arch: ppc64el, cc: gcc, container: 'dockcross/linux-ppc64le:latest', }